1. Field of the Invention
This invention relates to a shield for reducing the stray magnetic field in the vicinity of a magnetic resonance (MR) magnet.
2. Description of the Prior Art
An MR imaging device using an MR magnet is utilized in medical diagnostics in hospitals. The MR magnet generates a high intensity magnetic field. The disturbance to other hospital equipment due to the magnetic field generated by the MR magnet should be as low as possible. In order to reduce the intensity of the magnetic field outside the MR device, a magnetic shield is provided. The magnetic shield is also effective to generate a uniform magnetic field in the magnet working volume negating the effect of the surrounding structural elements.
Japanese Patent Disclosure (Kokai) No. 61-252613 and U.S. Pat. No. 4,646,045 disclose a magnetic shield for an MR magnet wherein a magnetic shield is provided in the form of a cylindrical shell of magnetic material surrounding the MR magnet. The cylindrical shell is situated so that its longitudinal axis is coaxial with the magnetic axis of the magnet. The cylindrical shell is fabricated with a number of staves. Two disk shape end caps of magnetic material are secured to either end of the cylindrical shell. The end caps each have a central aperture.
A magnetic shield, typically described above, with an MR magnet is a heavy object, generally weighing as much as 20 tons. However, the maximum loading for passageway floors in hospitals is generally no more than 10 tons. This makes it impossible to transport the MR magnet and its associated magnetic shield into the hospital room as a single unit. In the case of the conventional device described above, the method adopted is that the staves and the end caps are removed from the MR magnet and conveyed into the hospital individually for later assembly in the MR imaging device room in the hospital.
However, the room where the MR imaging device is installed is simply an ordinary hospital room, and no apparatus, such as a crane, is generally available for lifting very heavy objects. The unavoidable implication here is that when the magnetic shield is assembled in the MR device room, all the work has to be done by human muscle-power. In the conventional device described above, each stave weighs about a hundred and forty or fifty kilograms, which makes it impossible for them to be lifted and assembled into the magnetic shield by human muscle-power alone. This leads to an inescapable drawback, namely, that basically a magnetic shield for the MR magnet cannot be installed in a room in an existing hospital building.
The present method of dealing with this situation is to erect a new building specially for the MR imaging device, and lower the device, with its magnetic shield, down into the building by means of a large mobile crane while the building itself is under construction. However, in this case, the cost of construction may exceed the cost of the device itself, so that the total cost is exceedingly high. High cost is a major drawback inhibiting the installation of MR imaging devices.
In the case of the conventional device, moreover, the staves have to be cut away at the positions of the service port and the legs. Since the magnetic flux passes inside the staves in the axial direction of the magnetic shield, this has the effect that a magnetic circuit is formed, with magnetic flux leaking temporarily into the space where the staves have been cut, and subsequently being absorbed into the neighboring staves. The drawback which results is a marked deterioration in magnetic shielding efficiency.